I'm trying to build some cheap 2-way speakers.My receiver runs at 100 watts at 8 ohms, so does that mean I have to get a 4 ohm tweeter and woofer and wire them in series?I'm also having trouble finding a crossover. Do I have to find one that accepts 4 ohms for the tweeter and 4 ohms for the woofer or do I need one that accepts a total load of 8 ohms?Does it absolutely need to be rated to run at 100 watts or is there any wiggle room there?

Even a simple speaker is not as simple as it seems. Judging from your questions, it appears that this will be a first-time project for you. Also from your questions, it sounds like you might be on the wrong track to make something that sounds decent.

Question one:My receiver runs at 100 watts at 8 ohms, so does that mean I have to get a 4 ohm tweeter and woofer and wire them in series? No. Running the tweeter and woofer in series would likely be a complete disaster sound-wise. Also, most receivers can handle speakers somewhat below 8 ohms impedance. I wouldn't worry too much about impedance ratings at this point.

Question two: I'm also having trouble finding a crossover. Do I have to find one that accepts 4 ohms for the tweeter and 4 ohms for the woofer or do I need one that accepts a total load of 8 ohms? Finding an off-the-shelf, generic crossover that would work with just any pairing of woofer and tweeter will be next to impossible. The VAST majority of loudspeakers have crossovers that are tailor-made for the drivers used. It not only compensates for impedance differences in the drivers, it also corrects phase, works with the speaker cabinet volume and baffle dimensions, and more. You'd be FAR, FAR more likely to produce a good loudspeaker by building one of the pre-designed kits sold by Meniscus and elsewhere. The crossovers in the kits have been custom designed to integrate with the chosen drivers and cabinet configuration (ported vs sealed, baffle step).

Question three:Does it absolutely need to be rated to run at 100 watts or is there any wiggle room there? You can always run a higher wattage amp with any speaker. Just don't turn it up enough to damage the speaker itself. If you have a receiver less than the rated wattage of the loudspeaker, you just can't run it as loud as you like. In fact, running a weaker amp beyond where it clips can damage a loudspeaker. I'd say that for most speakers, a 100-watt rating is a nice place to be.

As Bill implied there are many very well received kits out there, starting at just over $100, sans box on this site. And a pre-designed kit is a great way to start the learning process while being confident you'll end up with great sound.

If, as is the case for some, you already have the drivers or have some in mind and want to roll your own from scratch, there are many helpful guides out there. These guides come in PDF, HTML and real human being formats. But be forewarned! The learning curve is steep, the process is long and for many, the whole thing can be addicting. =)